Over the last few years, semiconductor devices based on group 13 nitrides such as GaN have drawn considerable interest because of their promise in high-temperature and high-power applications, e.g. power and microwave amplification, RF signal processing and sensor applications.
A unique feature of group 13-nitrides like AlN, GaN and InN is the large difference in electronegativity. Nitrogen is the most electronegative group V element. This results in spontaneous polarization in the group 13 nitrides. In addition, strain in the crystal structure of these materials enhances the polarization effect. In GaN/AlGaN heterostructures, this additional piezoelectric polarization due to the differences in crystal lattice parameters gives rise to a two-dimensional electron gas formed at the heterojunction between these two layers. This two-dimensional electron gas forms the conducting channel between the source and drain of a normally-on high electron mobility transistor (HEMT).
It is important to create a low-ohmic contact to the two-dimensional electron gas in order to successfully utilize these devices in high-performance application domains. Most ohmic contacts on GaN/AlGaN heterostructures are based on Ti/Al-based metallization schemes. Titanium creates nitrogen vacancies by forming TiN, which enables electrons to tunnel to the two-dimensional electron gas underneath the AlGaN. Aluminium is present to react with Ti to prevent oxidation of the titanium.
Typically, gold is used as the bulk metal on top of the aluminium, and is separated from the aluminium by one or more diffusion barriers. Most frequently reported metallization schemes include Ti/Al/Ti/Au, Ti/Al/Ni/Au and Ti/Al/Pt/Au.
However, the use of gold adds significant cost to the manufacturing process of such semiconductor devices, not in the least because gold is not commonly used in existing manufacturing processes such as a CMOS process. This therefore requires significant redesign of the manufacturing process, which is undesirable from a cost perspective.
To be able to process group 13 nitride semiconductor devices such as GaN/AlGaN high-electron mobility transistors (HEMTs) on silicon substrates in a main-stream semiconductor fab, such as a silicon-based fab, gold has to be eliminated from the manufacturing process. It is however far from trivial to make this elimination without suffering an increase in the contact resistance of the ohmic contacts, thereby compromising the performance of the semiconductor device.
F. M. Mohammed et al. In Electr. Letters, 41 (17), 2005, pages 984-985, disclose a Ti/Si/Al/Si/Mo/Au layer stack as an ohmic contact for a AlGaN/GaN HEMT. Upon annealing such a contact at 850° C. for 30 seconds, it was found that the incorporation of the silicon layers in this stack caused the formation of silicided inter-metallics as well as the unintentional doping of the AlGaN/GaN heterostructures with silicon, which lead to a contact resistivity of 0.16 Ω·mm and a specific contact resistivity of 6.77×10−7 Ω·cm2.
However, this ohmic contact still relies on gold as the bulk metal, which is undesirable as previously explained. Moreover, it has been experimentally found, e.g. by the present inventors, that the presence of the Si layer in the complete bulk contact causes problems in terms of reproducibility of the quality of the contact in industry-scale manufacturing processes.